In recent years, swallowable capsule endoscopes are being developed in the field of endoscopes. The capsule endoscopes have an image pickup function and a radio communication function. While the capsule endoscope is swallowed through a subject's mouth and is ejected naturally in order to observe the insides of various internal organs, it moves along the insides of internal organs such as belly, small intestine and large intestine according to their peristaltic activities and simultaneously picks up images sequentially.
Image data, which are picked up by the capsule endoscope in the subject while the capsule endoscope is moving inside the organs, are sequentially transmitted to the outside of the subject by using radio signals and are stored in a memory provided into a receiver outside of the subject or are displayed on a display provided to the receiver. Doctors, nurses and the like can carry out diagnosis based on the images which are displayed on the display according to the image data stored in the memory or the images which are displayed on the display provided to the receiver at the same time of reception.
Generally as to the receiver, in the case where the image data are stored in the memory, a plurality of antennas for receiving image signals transmitted from the capsule endoscope are dispersively arranged outside the subject, and one antenna which receives image signals with less errors is selected so as to receive the images. For example, Patent Document 1 discloses a receiver which switches reception between plural antennas arranged outside a subject, and tracks a position of a capsule endoscope in the subject as a transmission source of image signals based on electric field strength received by the antennas.    Patent Document 1: Japanese Patent Application Laid-Open No. 2003-19111